I'm not sure if you're having the same problem as me but even though I had 5.1 Speakers set through Windows Sound options... my Center speaker sound was coming out of both front speakers and my Left Rear would come out from Left Front and Rear Right from Front Right.
I finally figured out that you also have to set 5.1 Speakers through the Creative Audio Console...which is usually under START >> PROGRAMS >> CREATIVE.
after that, all my speakers work correctly.
Also make sure you have that little switch on the back of the Logitech control unit the 3 plugs are plugged into is set to 6CH Direct...i forgot about that too when I set my speakers up.
Hope this helps!